Catalytic reforming, or hydroforming, is an established process in the petroleum refining industry and has been used for improving the octane quality of naphthas and straight run gasolines for many years. Catalysts used in catalytic reforming are recognized as dual- or poly-functional, the catalyst composite including a component comprising a metal, or metals, or a compound or compounds thereof, providing a hydrogenation-dehydrogenation (hydrogen transfer) function, isomerization function, hydrocracking function and/or hydrogenolysis function, and an acidic component providing isomerization, cracking, and/or hydrocracking functions.
The platinum group, or Group VIII noble metals (ruthenium, osmium, rhodium, iridium, palladium and platinum), despite their expense, have been long recognized as particularly efficient hydrogen transfer components. Platinum per se has, in fact, proven par excellent as a hydrogen transfer component and, in fact, possesses a combination of properties which makes it particularly suitable as a component for commercial reforming catalysts. Conventional reforming catalysts have thus long employed platinum composited with an inorganic oxide base, particularly alumina, to which halogen is added to supply the isomerization function. Platinum catalysts have achieved world-wide use in commercial reforming operations.
There is a desideratum in the art, occasioned in large part by the withdrawal of alkyl lead compounds based on ecological considerations, and intensive efforts are again underway to improve the octane quality of naphthas and gasolines while suppressing the use of such additives, or by elimination of such additives, and present emphasis is on the improvement of reforming catalysts. Improvements have been made, and new species of catalyts have been developed. The new family of reforming catalysts which have proven most satisfactory are largely bimetallic, or multimetallic, and most (if not all) include platinum as an essential component. Thus, platinum yet maintains a rank of distinction as a component of commercially viable reforming catalysts. The industry, in turning to catalysts which employ multimetallic components to provide effective hydrogen transfer and dehydrocyclization for improving the octane quality of naphthas and gasolines in commercial operations, has also utilized catalysts which contain both platinum and rhenium (e.g., U.S. Pat. No. 3,415,737). While the combination of platinum and rhenium has proven effective, surpassing platinum per se as a hydrogen transfer and dehydrocyclization component for commercial reforming operations, further improvements in the performance of such catalysts are quite desirable.
Platinum-rhenium catalysts possess satisfactory activity for use in reforming operations, activity being defined as that property which imparts the ability to produce aromatics, aromatic production (or octane improvement) generally being measured as a function of temperature, feed rate, etc. Platinum-rhenium catalysts also possess satisfactory selectivity which is defined as that property which imparts the ability of the catalyst to produce high yields of high octane number C.sub.5.sup.+ liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., C.sub.1 -C.sub.4 hydrocarbons, or solid carbonaceous by-products and coke, which form on the catalysts during reforming. These catalysts also possess good stability or activity maintenance, i.e., activity plotted as a function of time, good stability or activity maintenance being defined as high retention of activity and selectivity, or continued high activity and stability for prolonged periods during hydrocarbon conversion, or reforming operations.
While any commercially viable reforming catalyst must possess all of these properties to a significant degree, no catalyst used in real world operations can possess all of these properties to the ultimate degree. One of these characteristics may be possessed by a catalyst in admirable degree, but the proper quality of another of these characteristics may adversely affect the worth of the catalyst. Thus, a catalyst which possesses good selectivity does not necessarily have good activity, and vice versa. A small decrease in C.sub.5.sup.+ liquid yield can thus represent a large debit in commercial reforming operations. Conversely, the worth of a catalyst which possesses high selectivity may be jeopardized by the considerable capitol cost which necessitates large charges of noble metals containing catalysts. Proper balance between these several properties is essential in the commercial world and an improvement gained in one property, or characteristic, cannot be too much offset by loss of another if the catalyst is to prove commercially viable.
Platinum-rhenium catalysts possess a generally satisfactory balance between these several desirable properties for use in reforming, particularly as regards their selectivity, or ability to produce high C.sub.5.sup.+ liquid yields from naphthas and straight run gasolines. Nonetheless, quite obviously, further improved C.sub.5.sup.+ liquid yields are highly desired in the world's present short fuel supply situation. Hence, a goal of research and development is to further improve the performance of the basic platinum-rhenium catalyst, especially as relates to its selectivity.
Accordingly, it has now been discovered that a catalyst comprising catalytically active amounts of rhenium, especially platinum and rhenium, composited with a porous inorganic base, notably alumina, is more selective, and more stable for producing high octane products from gasolines and naphtha at reforming conditions if it is pretreated in a sequence which includes the steps of oxidation, dry hydrogen reduction, and sulfiding. The dry hydrogen reduction step is critical. In all embodiments, the hydrogen reduction step is conducted at conditions sufficient to remove product water from the catalyst as it is produced, and the reduction is continued until the stream of hydrogen gas leaving said catalyst (i.e., the exit gas) contains less than about 1000 parts per million of moisture (water) by volume, preferably less than about 500 ppm water. Whereas processes are known wherein oxidized rhenium-containing catalysts have been contacted with dry hydrogen, no one heretofore insofar as is known has recognized the necessity of continuing the reduction sufficient to remove all water, even the in situ water formed by the reduction of the metal oxides of the catalyst. In practicing the invention, the duration of contact of the catalyst with dry hydrogen is continued until the catalyst becomes dry, or desiccated; this state being reached when the hydrogen leaving said catalyst contains less than 1000 ppm water, preferably less than 500 ppm water. This means, of course, that the dry hydrogen used for the reduction must contain less than 1000 ppm moisture, or 500 ppm moisture, respectively; and preferably the hydrogen should be considerably drier, and should contain no more than 100 ppm water, preferably less than 50 ppm water.
The contact time and moisture level are to some extent interrelated, and several hours may be required to desiccate the catalyst adequately to properly reduce the catalyst. Thus, the period required to properly reduce the catalyst may range above about 2 hours, often above about 8 hours. However, it is feasible to achieve the desired reduction within a quite rapid period by the use of an initial high temperature oxidation treat to activate the porous inorganic oxide base, or alumina, such that its surface is thereafter substantially dry, and desiccated, and then to perform the hydrogen reduction step at a lower temperature than that employed to effect the oxidation. Thus, if the oxidation step is conducted at relatively high temperature, and the hydrogen reduction step is conducted at a temperature below that at which the oxidation step was conducted, the reduction time can be shortened considerably and highly active catalysts of consistently high selectivity and stability can be prepared as contrasted with similar catalysts conventionally prepared.
It is believed that the chief benefit imparted by the rhenium to the catalyst is that of greater coke tolerance. The mechanism which makes the catalyst more tolerant of the deactivating coke is not known with any degree of certainty, but it is believed that the formation of intermetallic catalytic alloys, or clusters, is a principal factor, and that the formation of these species is dependent on the existance thereof of rhenium and another metal, or metals, in a zero valent state. Consequently, it is believed that the process of this invention is one which provides greater effectiveness in the occurrence of a solid state reaction between rhenium and another metal, or metals, component.
In accordance therewith the pretreatment of the rhenium-containing catalysts includes three discrete steps, to wit: (1) the catalyst is contacted at an elevated temperature of at least about 850.degree. F. with an oxygen-containing gas, preferably air with or without added oxygen, the metal substitutent, or substitutents, constituting the hydrogenation-dehydrogenation component thereof being oxidized sufficient to form rhenium oxide, or rhenium oxide with other metal oxides, dispersed over the catalyst surface, and the oxidized catalyst is then (2) reduced with dry hydrogen, or a dry gas containing sufficient hydrogen to reduce the rhenium oxide, or rhenium oxide and other metal oxides, substantially to the zero valent state necessary for optimum intermetallic interaction. The temperature of the oxidation step (1), supra, should be greater than about 850.degree. F., and should be at least equal to or preferably greater than the temperature of the hydrogen reduction step. The hydrogen reduction step must be of sufficient duration to produce an exit gas containing less than about 1000 ppm water, preferably less than about 500 ppm water. If the temperature of the reduction and oxidation steps are about equal, then a relatively long reduction time is required to obtain optimum stability and selectivity advantages. If the temperature of the oxidation step is greater than the temperature of the reduction step, then the time required for the reduction can be greatly lessened. After the oxidation and reduction steps are completed (3) the catalyst is contacted with a sulfur-containing fluid, gas or liquid, to convert the reduced metal surface substitutents of the catalyst to the sulfide form.
In the oxidation step (1), supra, the catalyst is suitably contacted with a flowing gas stream of an oxygen-containing gas, preferably air, suitably at temperatures ranging from about 850.degree. F. to about 1100.degree. F., preferably at temperatures ranging from about 950.degree. F. to about 1050.degree. F. Suitably, the period of treatment ranges from about 2 hours to about 10 hours, preferably from about 3 hours to about 4 hours. In the reduction step (2), supra, the oxidized metal surface of the catalyst is suitably contacted with a flowing stream of dry hydrogen, or a dry hydrogen-containing gas, at a temperature below that at which the oxidation step was carried out, suitably at a temperature ranging from about 100 to about 250 Farenheit degrees, preferably from about 100 to about 150 Farenheit degress, below that at which the oxidation step was conducted. Expressed in absolute terms, the reduction is generally conducted at temperatures ranging from about 700.degree. F. to about 950.degree. F., preferably from about 800.degree. F. to about 950.degree. F. The time of reduction suitably ranges from about 0.5 hours to about 10 hours, though from about 0.5 hours to about 2 hours is generally sufficient for complete reduction of the metal oxides of the catalyst when adequate differential temperatures are maintained between the oxidation and reduction steps. Absolute pressures generally range from about atmospheric to about 400 pounds per square inch (psi), pressures ranging below about 100 psi being preferred.
The substantially complete reduction of the catalytic metal oxides to the zero valent state is essential. Again, whereas Applicants do not wish to be bound by a theory of mechanism, it is believed that the completeness of the reduction is controlled by the chemical equilibrium represented by the equation EQU 7H.sub.2 +Re.sub.2 O.sub.7 .revreaction.2Re+7H.sub.2 O
According to the principle of Le Chatlier the reduction may be driven to completion by providing a significantly higher partial pressure of the reactant hydrogen to that of the product water, by removing the product water from the reacting system as it is produced, or by a combination of both these factors. In essence it is desirable to maintain as dry a reducing environment as practical to favor the formation of rhenium metal. This dehydrated condition may be created in two ways. In the metal. This dehydrated condition may be created in two ways. In the case of long reduced catalysts this relationship is established by the continuous removal of water from the reaction with extended hydrogen flow. Alternatively, this relationship is established by the absorption of a critical amount of the product water by the alumina support. In this instance it is necessary that the alumina first become activated by a high temperature air treat, and second that the reduction be performed at a lower temperature than the air activation temperature so that the equilibrium for the desiccant action of the alumina is favored. Both of these procedures effectively remove water from the reaction, elevate the hydrogen to water partial pressure relationship, and drive the reduction to completion. In this way the level of dehydration of the two reductions becomes equal, the same degree of reduction occurs, and equivalent catalyst performance results. The effect of proper catalyst reduction, in any regard, is to decrease hydrocracking reactions and thereby increase liquid yields. Since hydrocracking sites are responsible for coking reactions as well, decreasing these sites favors catalytic stability.
The degree of catalyst reduction is important to ultimate catalyst performance. The reducibility of rhenium oxide in rhenium containing catalysts has been a topic of some controversy. McNicol [J. Catalysis, 46, 438 (1977)] has shown that a platinum-rhenium (Pt-Re) (0.4/0.2 wt. %) catalyst is &gt;80% reduced after 1 hour at 500.degree. C. and 95% reduced after 1 hour at 550.degree. C. Thus, it would seem that a typical Pt/Re reforming catalyst reduced in this way should perform satisfactorily for naphtha reforming. However, it has been shown that such catalysts reduced in this way are in fact inferior, and lack consistency in their ability to reform hydrocarbons. The critical element in the reduction process is not the time period per se, and reduction temperature and pressure are of secondary importance. What is most important, it has been found, is that the reduction be carried out in the essential absence of water, inclusive of that released in situ by the conversion of the metal oxides. The presence of water, it is believed, may stabilize rhenium in a high oxidation state and inhibit its reduction to the required low valence state.
A properly reduced rhenium catalyst will require optimum sulfiding conditions to yield a superior reforming catalyst. The reduced catalyst is sulfided by contact with a sulfide, generally hydrogen sulfide or compound which will produce hydrogen sulfide in situ. Suitably, the catalyst can be sulfided by contacting it with a sulfur-containing gas, especially hydrogen sulfide containing hydrogen or gas containing hydrogen and other gases in admixture with hydrogen sulfide or a hydrogen sulfide precursor in dilute concentration, i.e., from about 50 parts to about 5000 parts per million parts by volume, preferably from about 1000 to about 3000 ppm of hydrogen sulfide. The catalyst can also be sulfided by breakthrough sulfiding with hydrogen sulifde. Breakthrough sulfiding is generally conducted by sulfiding with hydrogen sulfide at a pressure equal to or less than the reduction pressure and at a temperature equal to or less than the reduction temperature to provide superior performance. Suitably also, the reduced catalyst can be presulfided by contact with a sulfur-containing naphtha feed. However, the reduction remains the critical feature independent of the sulfiding procedure employed.
The principal benefit of rhenium-containing catalysts properly oxidized, prereduced, and sulfided is increased liquid yield in naphtha reforming. It is found that C.sub.5.sup.+ liquid yield advantages of 4-5 LV% ca be provided by proper prereduction and sulfiding of these catalysts, subsequent to their oxidation. The yield advantage is provided by the suppression of metal promoted hydrogenolysis reactions which lead to the formation of C.sub.1 -C.sub.4 hydrocarbons, especially the C.sub.2 -C.sub.4 hydrocarbons. While hydrogenolysis of the methane producing type can generally be effectively controlled by conventional sulfiding, the type of hydrogenolysis which leads to the formation of C.sub.2 -C.sub.4 hydrocarbons is suppressed by proper oxidation, prereduction and sulfiding. In addition, catalyst pretreatment in accordance with this invention affords greater aromatization activity.
This method of pretreatment is applicable to virtually any rhenium containing catalyst. A catalyst composition particularly suitable for pretreatment in accordance with this invention is one which comprises from about 0.05 to about 3 percent platinum, preferably from about 0.1 to about 1 percent platinum, and from about 0.05 to about 3 percent rhenium, generally from about 0.1 to about 1 percent rhenium, based on the total weight (dry basis) of the composition. Preferably, the rhenium contained in such catalyst compositions ranges from about 0.1 to about 1 percent, more preferably from about 0.15 to about 0.70 percent, based on the weight (dry basis) of the total catalyst compositions.
The preferred catalyst of this invention not only includes platinum and rhenium, but platinum, rhenium and a promoter metal, suitably palladium or copper or both. The preferred catalyst also contains halogen. The catalyst compositions initially contain from about 0.1 to about 5 percent halogen, preferably from about 0.5 to about 2.5 percent halogen. The catalysts, as a result of the presulfiding also contain sulfur suitably from about 0.001 to about 2 percent, preferably from about 0.001 to about 0.15 percent sulfur, based on the total weight (dry basis) of the catalyst compositions.
The metals hydrogenation-dehydrogenation components are composited with a support material, suitably mildly or moderately acidic refractory inorganic oxides, e.g., silica, silica-alumina, magnesia, thoria, boria, titania, zirconia, various spinels and the like, including, in particular, alumina, and more particularly gamma alumina, which species are preferred. High surface area catalysts, or catalysts having surface areas ranging upwardly from about 10, and preferably 100 m.sup.2 /g. (B.E.T.) are preferred. In particular, catalysts having surface areas ranging from about 150 to about 600 m.sup.2 /g. prove quite satisfactory.
The platinum and rhenium, or platinum, rhenium and other components can be composited or intimately associated with the porous inorganic oxide support by various techniques known to the art such as ion-exchange, coprecipitation, or physical mixing of fine particles with the alumina in the sol or gel form, etc. For example, the catalyst composite can be formed by adding together suitable reagents such as water soluble salts of platinum and rhenium, and ammonium hydroxide or ammonium carbonate, and a salt of aluminum such as aluminum nitrate, aluminum sec-butoxide, or aluminum sulfate to form aluminum hydroxide. When the metals are included in the preparation of the support, higher concentrations of the metals are often necessary. The aluminum hydroxide containing the salts of platinum and rhenium can then be heated, dried, formed into pellets or extruded, and then calcined in nitrogen or nonagglomerating atmosphere. The platinum can be added to the support, and then the rhenium added. Or the rhenium can be added, and then the platinum added. Suitably, platinum, rhenium, and halogen are added simultaneously and the catalyst then dried. On the other hand, if the platinum, and rhenium have already been added as by co-precipitation from an alumina gel, the halide can be subsequently added, e.g., by impregnation from an aqueous HCl solution, or by gas phase chlorination with an HCl-Cl.sub.2 mixture or other halide precursor.
Essentially any hydrocarbon fraction containing paraffins, naphthenes, and the like, admixed one with the other or in admixture with other hydrocarbons, can be converted by means of the catalyst of this invention. A suitable feed can be a virgin naphtha, cracked naphtha, a Fischer-Tropsch naphtha or the like, or mixtures thereof. The feed is contacted at reforming conditions in the presence of hydrogen (once-through, or recycle) with a catalyst composite including a support which contains catalytically active amounts of the metals. Typical feed stream hydrocarbon molecules are those containing from about 5 to about 12 carbon atoms, or more preferably from about 6 to about 12 carbon atoms, or more preferably from about 7 to about 10 carbon atoms. Naphthas, or petroleum fractions, boiling within the range of from about 80.degree. F. to about 450.degree. F., and preferably from about 125.degree. F. to about 375.degree. F., contain hydrocarbons or carbon numbers within these ranges. Typical fractions thus usually contain from about 20 to about 80 volume percent of paraffins, both normal and branched, which fall in the range of about C.sub.5 to C.sub.12, and from about 20 to about 80 volume percent of naphthenes boiling within the range of about C.sub.6 to C.sub.12. Typical feeds generally contain from about 5 through about 20 volume percent of aromatics which boil within the range of about C.sub.6 to C.sub.12, typically as produced in the product from the naphthenes and paraffins.
The invention will be more fully understood by reference to the following demonstrations, selected nonlimiting examples, and comparative data which illustrate its more salient features. All parts are given in terms of weight except as otherwise specified.
The demonstrations represented by Runs 1-6 below show that long reduction times with dry hydrogen provides superior catalysts as contrasted with catalysts otherwise similarly prepared except that the reduction periods are of short duration. In these runs the moisture level of the exit hydrogen gas was not measured, and at the time the critical nature of the moisture contained in the exit gas was not recognized.